A magnetic tunnel junction (MTJ) device is comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. One of the ferromagnetic layers has a higher saturation field in one direction of an applied magnetic field, typically due to its higher coercivity than the other ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed primarily as memory cells for solid state memory. The state of the MTJ memory cell is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of the two ferromagnetic layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the ferromagnetic layers, for example, a layer whose magnetic moment is fixed or prevented from rotation, is predominantly composed of electrons of one spin type (spin up or spin down, depending on the orientation of the magnetic moment of the ferromagnetic layer). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material comprising the ferromagnetic layer at the interface of the ferromagnetic layer with the tunnel barrier layer. The first ferromagnetic layer thus acts as a spin filter. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the second ferromagnetic layer. Usually, when the magnetic moment of the second ferromagnetic layer is parallel to the magnetic moment of the first ferromagnetic layer, there are more available electronic states than when the magnetic moment of the second ferromagnetic layer is aligned antiparallel to that of the first ferromagnetic layer. Thus, the tunneling probability of the charge carriers is highest when the magnetic moments of both layers are parallel, and is lowest when the magnetic moments are antiparallel. When the moments are arranged neither parallel nor antiparallel, the tunneling probability takes an intermediate value. Thus, the electrical resistance of the MTJ memory cell depends on the spin polarization of the electrical current and the electronic states in both of the ferromagnetic layers. As a result, the two possible magnetization directions of the ferromagnetic layer whose magnetization direction is not fixed uniquely define two possible bit states (0 or 1) of the memory cell. Although the possibility of MTJ memory cells has been known for some time, serious interest has lagged because of difficulties in achieving responses of the magnitude predicted in practical structures and at noncryogenic temperatures.
A magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor, such as that used as a MR read head for reading data in magnetic recording disk drives, operates on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy (Ni.sub.81 Fe.sub.19). A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the disk in a disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element and a corresponding change in the sensed current or voltage.
The use of a MTJ device for a memory application has been proposed, as described in U.S. Pat. No. 5,640,343 and IBM's U.S. Pat. No. 5,650,958. The use of a MTJ device as a MR read head has also been proposed, as described in U.S. Pat. No. 5,390,061. One of the problems with such MTJ devices is that the magnetizations of the ferromagnetic layers generate magnetic dipolar fields. This leads to a magnetostatic interaction between the ferromagnetic layers within a particular device and to magnetostatic interactions between MTJ devices in an array of such MTJ devices, such as an array of MTJ memory cells proposed for a nonvolatile magnetic memory array. The magnetostatic interaction become increasingly important as the cross-sectional area of the MTJ device or devices is reduced. For memory applications the magnetostatic interactions between neighboring MTJ memory cells means that the properties of an individual MTJ memory element will be affected by the state of the neighboring MTJ memory elements. This will limit the packing density of the MTJ elements in an array since neighboring MTJ memory elements have to be spaced further apart than would otherwise be required. For MR read head applications the magnetostatic interaction between the ferromagnetic layers within the MTJ device limits the performance of the device. In addition, the magnetization of the flux sensing layer in the MR read head has to be progressively reduced as the data storage capacity of the magnetic recording system is increased to obtain the best performance.
In U.S. Pat. Nos. 5,477,482 and 5,541,868 it is proposed that giant magnetoresistance (GMR) elements in the shape of hollow cylinders or hollow washers be used to reduce magnetostatic interactions between elements in a magnetic memory array of GMR elements. This method is not useful for MR read head applications. Moreover this method introduces considerable complexity into the fabrication of the individual magnetic elements. Furthermore these elements are of necessity large since the elements will comprise several minimum-area lithography squares in order to create elements with hollow interiors. Thus these elements are not suitable for high density memory applications.
What is needed is a MTJ device in which the ferromagnetic layers have reduced net magnetic moments such that the magnetostatic interactions between these layers within a single device (for MR read head applications) or between MTJ devices (for memory applications) can be reduced in a controlled manner without significantly increasing the complexity of fabrication of the MTJ device.